Rotary lawn mowers, having a blade rotated in a generally horizontal plane, about a central, generally vertical rotatable shaft, are known. The blade of such a lawn mower is a generally flat elongated piece of steel which is rotationally symmetric with the rotatable shaft. The blade has a sharpened edge on the leading edge of the outer portion of the blade. The outer portion trailing edge is generally slightly raised to cause a fan effect so as to lift the grass and to blow the clippings into a bag.
The cutting edge of a prior art mower blade becomes dull very quickly. While the mower blade appears to be cutting for a long period of time, in actuality the cutting edge may have ceased to be a true cutting edge and may thus sever blades of grass only because the mower blade is rotating very fast. Thus, the blades of grass are broken by the spinning mower blade and are bruised in the process. The grass develops a brown tip because of the bruising, has an unpleasing appearance, and is a damaged plant. Furthermore, since the dull mower blade must break the grass blades instead of cutting them, the motor which rotates the blade is under an increased load, thus requiring more maintenance and consuming more fuel.
Sharpening a mower blade entails removing the entire blade, grinding or filing a new edge on the blade, and remounting the blade. This procedure is difficult and time consuming.